Generally, in laser transmission welding two workpiece parts are connected to one another. For this purpose, the two workpiece parts must directly contact in the joining region so that they can be welded. One of the workpiece parts is transmissive for the laser radiation and faces the laser source during welding. The other workpiece part is absorptive for the laser radiation. The laser beams penetrate the transmissive workpiece part and are converted into heat energy in the upper layers of the absorptive workpiece part. The upper layers of the absorptive workpiece part are fused which, due to heat conduction, also brings about a fusing of the adjoining layers of the transmissive workpiece part. The two melts can merge with each other and bond the two workpiece parts in cooled condition by means of a weld to form a welded workpiece.
For the weld to form homogeneously and with high strength, it must be ensured that the process of heat conduction can take place without interruption over the course of the weld seam. This requires direct contact between the workpiece parts in the joining region. Contact can be produced in a highly reliable manner by pressing the two workpiece parts against one another by suitable devices during welding.
A clamping device disclosed in Laid Open Application DE 10 2007 042 739 A1 is used for laser transmission welding to press a transmissive lid onto a housing-like bottom part. The absorptive bottom part has an opening which must be closed by the exactly fitting lid. To this end, the plane edge of the lid lies on the edge of the opening. The edges lying one on top of the other form the joining region and are to be joined by a circumferential weld. In order to apply a pressing force in the joining region, a pressing body (in this case, clamp chucks) is placed on the lid within the circumferential weld seam. The shape of the pressing body is adapted to the shape of the lid so that its contact surface reaches as close as possible to the joining region and the pressing force can be introduced so as to be evenly distributed in the lid.
The pressing body is connected via four connecting struts to an annular carrier (in this case, clamping ring) of the device, this carrier being guided outwardly around the circumferential weld seam. The pressing force is transmitted from the carrier to the pressing body via the connecting struts. An intermediate space remains between the carrier and the pressing body through which the laser beam can be directed along the weld seam onto the workpiece. The intermediate space is interrupted only by the connecting struts. The cross section of the connecting struts is as small as possible so as to minimize shadowing of the laser beams. The laser beam is guided circumferentially along the weld seam by a scanner system or by a movable welding head.
In an embodiment of the device, the pressing of the lid by means of the pressing body inside the weld seam is combined with a clamping element additionally fastened to the carrier outside of the weld seam. This assumes that there will still be sufficient application surface at the edge of the lid for applying the clamping element. A weld seam which connects the lid to the bottom part up to the outermost edge is thus no longer possible.
This device is disadvantageous in that, in order to introduce the pressing force, it requires a suitable, stable and, therefore, specially fabricated lid which must be exactly aligned and fitted on either manually or by handling devices. The device is not intended, nor is it suitable, for handling a lid comprising thin sheet.
It must be assumed based on the construction that the clamping device is preferably used on small workpieces with simple weld seams. With workpieces in which the laser beam must be guided over long weld seams or over a number of winding weld seams, the machining time for feeding the laser increases appreciably. In workpieces having a plurality of adjacent circumferential weld seams, a corresponding quantity of pressing bodies would also be required. Attaching the pressing bodies by means of a corresponding quantity of connecting struts would require considerable effort and would inevitably either result in increased shadowing of the laser beam or the clamping device would be too unstable to transmit pressing forces.
In a device for through-transmission welding of two flat workpiece parts in the not-prior-published document DE 10 2011 055 203.0, these drawbacks are avoided by gluing the pressing bodies to a transparent carrier plate. This greatly facilitates the mounting of pressing bodies at the carrier plate, including pluralities of pressing bodies and pressing bodies of different shapes. The carrier plate is transparent to the laser radiation so that it can impinge on the workpiece through the intermediate spaces remaining between the pressing bodies without being interrupted by connecting struts. The carrier plate can be adapted very easily to different pressing forces or workpiece sizes by appropriate dimensioning of the carrier plate. The laser radiation source is a linear array which is composed of individually controllable diode laser emitters and which can emit the laser radiation onto the workpiece in the shape of a closed line when needed. Complete welding requires only a complete relative movement between the workpiece and the laser radiation source. Accordingly, only a very short time is needed to weld the entire workpiece. In areas which are not to be welded, either the corresponding diode laser emitters are switched off or the workpiece parts are covered by the pressing bodies.
The device is also suitable for welding sheets to absorptive workpiece parts with a shell-like construction and with thin wall structures standing upright in Z direction. To this end, the sheet is held by its edges and pre-loaded in X direction and Y direction. Pre-loaded in this way, it is then laid upon the shell-shaped workpiece part accompanied by light pressing force so that the end faces of the wall structures facing in Z direction contact the sheet. The welding process is then carried out.
During welding, the end faces of the wall structures are joined to the sheet. For this purpose, at least one outer pressing body and one inner pressing body are positioned at the level of the sheet on both sides of the end faces, but without exerting pressure on the sheet in Z direction. The pressing bodies which are oriented substantially perpendicular to the sheet form a defined intermediate space between the inner pressing body and outer pressing body through which the laser beam is guided onto the end faces. The intermediate space is formed in direction of the laser beam with the necessary length for homogenizing a laser beam entering in a divergent manner into the intermediate space by means of multiple reflections at the walls of the pressing bodies. The homogenized laser beam impinges on the end face and forms a two-dimensional weld seam. The homogenization achieves a strength which would not be possible with conventional laser transmission welding methods even in thin wall structures with correspondingly small end faces.
However, because the sheet is tensioned in X direction and Y direction, the device described in the not-prior-published DE 10 2011 055 203.0 is only able to compensate for tolerances in the planarity of the end walls of the absorptive workpiece part to a limited extent.
Further, for certain applications of shell-like workpieces such as receptacles, for example, it is of particular interest that the wall structures of the receptacle be as thin as possible. An example of such an application is a heat exchanger in which the heat transfer is to be carried out via very thin inner dividing walls or via the thin sheet serving to close the receptacle opening. However, it would be impossible to weld even thinner wall structures because if the end face were any smaller it would no longer be possible to achieve adequate strength in the weld seam.